Insulating sheet and laminate

ABSTRACT

Provided is an insulating sheet capable of effectively enhancing thermal conduction and adhesiveness and effectively suppressing variation in the adhesiveness. The insulating sheet according to the present invention contains a thermosetting component and boron nitride, the insulating sheet has a first surface on one side in a thickness direction and a second surface on the other side in the thickness direction, and a first average aspect ratio of the boron nitride in a region having a thickness of 10% of a thickness of the sheet, from the first surface toward the second surface is smaller than a second average aspect ratio of the boron nitride in a region having a thickness of 90% of a thickness of the sheet, from the second surface toward the first surface.

TECHNICAL FIELD

The present invention relates to an insulating sheet containing athermosetting component and boron nitride. The present invention alsorelates to a laminate using the insulating sheet.

BACKGROUND ART

Electronic and electrical apparatuses have recently been downsized andallowed to have higher performance, and thus electronic components havebeen mounted with a higher package density. Thus, how to dissipate heatgenerated from the electronic component in a narrow space is a problem.Since the heat generated from the electronic component is directlylinked to reliability of electronic and electrical apparatuses,efficient dissipation of the generated heat is an urgent issue.

As one means for solving the above problems, there is a means using aceramic substrate having high thermal conduction as a heat dissipationsubstrate on which a power semiconductor device or the like is mounted.Examples of such a ceramic substrate include an alumina substrate and analuminum nitride substrate.

However, the means using a ceramic substrate has problems that it isdifficult to form a multilayer, processability is poor, and the cost isvery high. In addition, since a difference in linear expansioncoefficient between the ceramic substrate and a copper circuit is large,there is also a problem that the copper circuit tends to peel off duringa cooling and heating cycle.

Thus, a resin composition using boron nitride having a low linearexpansion coefficient, in particular, hexagonal boron nitride hasattracted attention as a heat dissipation material. A crystal structureof hexagonal boron nitride is a layered structure of a hexagonal networksimilar to graphite, and a particle shape of hexagonal boron nitride isscaly. Thus, it is known that hexagonal boron nitride has a propertythat the thermal conductivity in the plane direction is higher than thethermal conductivity in the thickness direction, and the thermalconductivity is anisotropic. The resin composition described above maybe used by being formed into a resin sheet or the like.

As an example of a resin sheet containing boron nitride, the followingPatent Document 1 discloses a multilayer resin sheet having a resincomposition layer and an adhesive layer. The resin composition layercontains a thermosetting resin and a filler. The adhesive layer isdisposed on at least one surface of the resin composition layer.Arithmetic average roughness Ra of the adhesive layer at a surface thatdoes not face the resin composition layer is 1.5 μm or less. The fillerincludes a boron nitride filler.

RELATED ART DOCUMENT Patent Document

-   Patent Document 1: JP 2013-039834 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The conventional resin sheet containing boron nitride as described inPatent Document 1 may be laminated on copper foil, a metal plate, or thelike to be used as a laminate.

In the conventional resin sheet containing boron nitride as described inPatent Document 1, although the thermal conduction can be enhancedbecause boron nitride is used, it is difficult to enhance adhesivenessbetween the resin sheet and the copper foil. In the conventional resinsheet containing boron nitride, it is difficult to achieve both thethermal conduction and the adhesiveness.

Furthermore, in the conventional resin sheet containing boron nitride,an area of an end surface on which a relatively large number offunctional groups of boron nitride are present may be small. As aresult, the adhesiveness between the resin sheet and the copper foil mayvary.

An object of the present invention is to provide an insulating sheetcapable of effectively enhancing thermal conduction and adhesiveness andcapable of effectively suppressing variation in the adhesiveness. It isalso an object of the present invention to provide a laminate using theinsulating sheet.

Means for Solving the Problems

According to a broad aspect of the present invention, an insulatingsheet containing a thermosetting component and boron nitride isprovided. The insulating sheet has a first surface on one side in athickness direction and a second surface on the other side in thethickness direction, and a first average aspect ratio of the boronnitride in a region having a thickness of 10% of a thickness of thesheet, from the first surface toward the second surface is smaller thana second average aspect ratio of the boron nitride in a region having athickness of 90% of a thickness of the sheet, from the second surfacetoward the first surface.

In a specific aspect of the insulating sheet according to the presentinvention, an absolute value of a difference between the first averageaspect ratio and the second average aspect ratio is 1 or more and 20 orless.

In a specific aspect of the insulating sheet according to the presentinvention, the first average aspect ratio is 2 or more and 20 or less.

In a specific aspect of the insulating sheet according to the presentinvention, an average major diameter of the boron nitride of the entireinsulating sheet is 1 μm or more and 40 μm or less.

In a specific aspect of the insulating sheet according to the presentinvention, a content of the boron nitride is 20% by volume or more and80% by volume or less in 100% by volume of the insulating sheet.

According to a broad aspect of the present invention, there is provideda laminate including a thermal conductor, an insulating layer laminatedon one surface of the thermal conductor, and a conductive layerlaminated on a surface of the insulating layer opposite to the thermalconductor. In this laminate, a material of the insulating layer is theinsulating sheet described above.

Effect of the Invention

The insulating sheet according to the present invention contains thethermosetting component and boron nitride. The insulating sheetaccording to the present invention has the first surface on one side inthe thickness direction and the second surface on the other side in thethickness direction. In the insulating sheet according to the presentinvention, the first average aspect ratio of the boron nitride in theregion having a thickness of 10% of the thickness of the sheet, from thefirst surface toward the second surface is smaller than the secondaverage aspect ratio of the boron nitride in the region having athickness of 90% of the thickness of the sheet, from the second surfacetoward the first surface.

Since the insulating sheet according to the present invention isprovided with the above-mentioned configuration, thermal conduction andadhesiveness can be effectively enhanced, and variation in theadhesiveness can be effectively suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an insulatingsheet according to one embodiment of the present invention.

FIG. 2 is a cross-sectional view schematically showing a laminateobtained using the insulating sheet according to one embodiment of thepresent invention.

FIG. 3 is a schematic view for explaining each region for which anaverage aspect ratio of boron nitride is to be obtained in theinsulating sheet according to the present invention.

FIG. 4 is a schematic view for explaining each region for which theaverage aspect ratio of boron nitride is to be obtained in theinsulating sheet according to the present invention.

MODE(S) FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

(Insulating Sheet)

An insulating sheet according to the present invention contains athermosetting component and boron nitride. The insulating sheetaccording to the present invention has the first surface on one side inthe thickness direction and the second surface on the other side in thethickness direction. In the insulating sheet according to the presentinvention, the first surface and the second surface face each other. Inthe insulating sheet according to the present invention, a first averageaspect ratio of the boron nitride in a region having a thickness of 10%of a thickness of the sheet, from the first surface toward the secondsurface is smaller than a second average aspect ratio of the boronnitride in a region having a thickness of 90% of a thickness of thesheet, from the second surface toward the first surface.

Since the insulating sheet according to the present invention isprovided with the above-mentioned configuration, thermal conduction andadhesiveness can be effectively enhanced, and variation in theadhesiveness can be effectively suppressed.

In the insulating sheet according to the present invention, the averageaspect ratio of boron nitride in a region near the first surface of theinsulating sheet is relatively small, and an area of an end surface ofboron nitride is relatively large. Functional groups such as hydroxyland amino groups are present on the end surface, and in boron nitride inthe region near the first surface of the insulating sheet, the amount offunctional groups such as hydroxyl and amino groups contributing toadhesiveness can be increased. As a result, when a conductive layer suchas copper foil is laminated on the first surface of the insulatingsheet, it is possible to effectively enhance adhesiveness between theinsulating sheet and the conductive layer and to effectively suppressvariation in the adhesiveness between the insulating sheet and theconductive layer. Furthermore, the thermal conduction of the insulatingsheet can be more effectively enhanced by increasing the average aspectratio of boron nitride in a region other than the region near the firstsurface of the insulating sheet.

In the present invention, a first average aspect ratio of the boronnitride in the region (R1) having a thickness of 10% of the thickness ofthe sheet, from the first surface toward the second surface is smallerthan a second average aspect ratio of the boron nitride in the region(R2) having a thickness of 90% of the thickness of the sheet, from thesecond surface toward the first surface. The region (R1) is a regionbetween a first surface 1 a and a broken line L1 in FIG. 3. The region(R2) is a region between a second surface 1 b and the broken line L1 inFIG. 3.

The first average aspect ratio of boron nitride in the region (R1) ispreferably 2 or more, more preferably 3 or more, further preferably 4 ormore, and preferably 20 or less, more preferably 12 or less, furtherpreferably 10 or less, particularly preferably 8 or less. When the firstaverage aspect ratio is in the range from the above lower limit to theabove upper limit inclusive, the thermal conduction and the adhesivenesscan be more effectively enhanced, and variation in the adhesiveness canbe more effectively suppressed.

The second average aspect ratio of boron nitride in the region (R2) ispreferably 6 or more, more preferably 8 or more, further preferably 10or more, and preferably 25 or less, more preferably 23 or less, furtherpreferably 21 or less. When the second average aspect ratio is in therange from the above lower limit to the above upper limit inclusive, thethermal conduction and the adhesiveness can be more effectivelyenhanced, and variation in the adhesiveness can be more effectivelysuppressed.

An absolute value of a difference between the first average aspect ratioand the second average aspect ratio is preferably 1 or more, morepreferably 2 or more, further preferably 3 or more, and preferably 20 orless, more preferably 15 or less, further preferably 13 or less. Whenthe absolute value of the difference between the first average aspectratio and the second average aspect ratio is in the range from the abovelower limit to the above upper limit inclusive, the thermal conductionand the adhesiveness can be more effectively enhanced, and variation inthe adhesiveness can be more effectively suppressed.

The first average aspect ratio of the boron nitride in the region (R1)having a thickness of 10% of the thickness of the sheet, from the firstsurface toward the second surface is preferably smaller than a thirdaverage aspect ratio of the boron nitride in a region (R3) having athickness of 80% of the thickness of the sheet, that is from a position1/10 of the thickness of the sheet to a position 9/10 of the thicknessof the sheet from the first surface toward the second surface. Theregion (R1) is a region between the first surface 1 a and a broken lineL1 in FIG. 4. The region (R3) is a region between the broken line L1 anda broken line L2 in FIG. 4.

The third average aspect ratio of boron nitride in the region (R3) ispreferably 6 or more, more preferably 8 or more, further preferably 10or more, and preferably 25 or less, more preferably 23 or less, furtherpreferably 21 or less. When the third average aspect ratio is in therange from the above lower limit to the above upper limit inclusive, thethermal conduction and the adhesiveness can be more effectivelyenhanced, and variation in the adhesiveness can be more effectivelysuppressed.

An absolute value of a difference between the first average aspect ratioand the third average aspect ratio is preferably 1 or more, morepreferably 2 or more, further preferably 3 or more, and preferably 20 orless, more preferably 15 or less, further preferably 13 or less. Whenthe absolute value of the difference between the first average aspectratio and the third average aspect ratio is in the range from the abovelower limit to the above upper limit inclusive, the thermal conductionand the adhesiveness can be more effectively enhanced, and variation inthe adhesiveness can be more effectively suppressed.

A fourth average aspect ratio of the boron nitride in a region (R4)having a thickness of 10% of the thickness of the sheet, from the secondsurface toward the first surface is preferably smaller than the thirdaverage aspect ratio of the boron nitride in the region (R3) having athickness of 80% of the thickness of the sheet, that is from a position1/10 of the thickness of the sheet to a position 9/10 of the thicknessof the sheet from the first surface toward the second surface. Theregion (R4) is a region between the second surface 1 b and the brokenline L2 in FIG. 4. The region (R3) is a region between the broken lineL1 and a broken line L2 in FIG. 4.

The fourth average aspect ratio of boron nitride in the region (R4) ispreferably 2 or more, more preferably 3 or more, further preferably 4 ormore, and preferably 12 or less, more preferably 10 or less, furtherpreferably 8 or less. When the fourth average aspect ratio is in therange from the above lower limit to the above upper limit inclusive, thethermal conduction and the adhesiveness can be more effectivelyenhanced, and variation in the adhesiveness can be more effectivelysuppressed.

An absolute value of a difference between the fourth average aspectratio and the third average aspect ratio is preferably 1 or more, morepreferably 2 or more, further preferably 3 or more, and preferably 20 orless, more preferably 15 or less, further preferably 13 or less. Whenthe absolute value of the difference between the fourth average aspectratio and the third average aspect ratio is in the range from the abovelower limit to the above upper limit inclusive, the thermal conductionand the adhesiveness can be more effectively enhanced, and variation inthe adhesiveness can be more effectively suppressed.

The first average aspect ratio of boron nitride in the region (R1) canbe calculated from an electron microscope image of a cross section of asheet or a laminate produced by pressing the insulating sheet. Thesecond average aspect ratio of boron nitride in the region (R2) can becalculated from an electron microscope image of a cross section of asheet or a laminate produced by pressing the insulating sheet. The thirdaverage aspect ratio of boron nitride in the region (R3) can becalculated from an electron microscope image of a cross section of asheet or a laminate produced by pressing the insulating sheet. Thefourth average aspect ratio of boron nitride in the region (R4) can becalculated from an electron microscope image of a cross section of asheet or a laminate produced by pressing the insulating sheet.

The present specification also provides an insulating sheet having thefollowing first to third configurations.

First configuration: One insulating sheet contains a thermosettingcomponent and boron nitride.

Second configuration: The insulating sheet has a first surface on oneside in a thickness direction and a second surface on the other side inthe thickness direction.

Third configuration: A first average aspect ratio of the boron nitridein a region having a thickness of 10% of the thickness of the sheet,from the first surface toward the second surface is smaller than a thirdaverage aspect ratio of the boron nitride in a region having a thicknessof 80% of the thickness of the sheet, that is from a position 1/10 ofthe thickness of the sheet to a position 9/10 of the thickness of thesheet from the first surface toward the second surface.

In the insulating sheet having the above first to third configurations,the configuration in which the first average aspect ratio of the boronnitride in the region having a thickness of 10% of the thickness of thesheet, from the first surface toward the second surface is smaller thana second average aspect ratio of the boron nitride in a region having athickness of 90% of the thickness of the sheet, from the second surfacetoward the first surface is not necessarily provided.

(Boron Nitride)

The insulating sheet according to the present invention contains boronnitride. The boron nitride is not particularly limited, and examplesthereof include hexagonal boron nitride, cubic boron nitride, boronnitride prepared by a reduction-nitridation method using a boroncompound and ammonia, boron nitride prepared from a boron compound and anitrogen-containing compound such as melamine, and boron nitrideprepared from sodium borohydride and ammonium chloride. From theviewpoint of more effectively enhancing the thermal conduction, theboron nitride is preferably hexagonal boron nitride.

The boron nitride may constitute boron nitride agglomerated particles.The boron nitride agglomerated particles are agglomerated particlesformed by aggregation of boron nitride (primary particles).

The primary particles constituting the boron nitride agglomeratedparticles may be scaly or in a bent shape.

From the viewpoint of more effectively enhancing the thermal conductionand the adhesiveness and more effectively suppressing variation in theadhesiveness, an average aspect ratio of the boron nitride of the entireinsulating sheet is preferably 2 or more and more preferably 4 or more,and preferably 25 or less and more preferably 20 or less. When the boronnitride constitutes the boron nitride agglomerated particles, theaverage aspect ratio of the boron nitride means the average aspect ratioof boron nitride (primary particle) constituting the boron nitrideagglomerated particles.

The aspect ratio of the boron nitride represents a major diameter/minordiameter. The average aspect ratio of the boron nitride is obtained bymixing the boron nitride or the primary particles constituting the boronnitride agglomerated particles with thermosetting resin or the like,measuring the major diameter/minor diameter of each of 50 boron nitrideparticles (primary particles) randomly selected from an electronmicroscope image of a cross section of a sheet or a laminate produced bypressing, and calculating an average value. When the primary particlehas a bent shape, the primary particle is divided into two parts at abending point, and the major diameter is measured for the two dividedparts. The major diameter/minor diameter calculated from the part with alarger major diameter is taken as the major diameter/minor diameter ofthe primary particle.

From the viewpoint of more effectively enhancing the thermal conductionand the adhesiveness and more effectively suppressing variation in theadhesiveness, an average major diameter of the boron nitride of theentire insulating sheet is preferably 1 μm or more and more preferably 2μm or more, and preferably 40 μm or less and more preferably 35 μm orless. When the boron nitride constitutes the boron nitride agglomeratedparticles, the average major diameter of the boron nitride means theaverage major diameter of boron nitride (primary particle) constitutingthe boron nitride agglomerated particles.

The average major diameter of the boron nitride is obtained by mixingthe boron nitride or the primary particles constituting the boronnitride agglomerated particles with thermosetting resin or the like,measuring the major diameter of each of 50 boron nitride particles(primary particles) randomly selected from an electron microscope imageof a cross section of a sheet or a laminate produced by pressing, andcalculating an average value. When the primary particle has a bentshape, the primary particle is divided into two parts at a bendingpoint, and the major diameter is measured for the two divided parts. Themajor diameter of the part with a larger major diameter is taken as themajor diameter of the primary particle.

The content of the boron nitride in 100% by volume of the insulatingsheet is preferably 20% by volume or more, more preferably 25% by volumeor more, still more preferably 30% by volume or more, and particularlypreferably 40% by volume or more, and preferably 80% by volume or less,more preferably 75% by volume or less, still more preferably 70% byvolume or less, and particularly preferably 65% by volume or less. Whenthe content of the boron nitride is in the range from the above lowerlimit to the above upper limit inclusive, the thermal conduction and theadhesiveness can be more effectively enhanced.

(Thermosetting Component: Thermosetting Compound)

The insulating sheet according to the present invention contains athermosetting component. The thermosetting component preferably containsa thermosetting compound, and preferably contains a thermosetting agent.The thermosetting component preferably contains the thermosettingcompound and the thermosetting agent. The thermosetting compound is notparticularly limited. Examples of the thermosetting compound includestyrene compounds, phenoxy compounds, oxetane compounds, epoxycompounds, episulfide compounds, (meth)acrylic compounds, phenolcompounds, amino compounds, unsaturated polyester compounds,polyurethane compounds, silicone compounds and polyimide compounds. Onekind of the thermosetting compound may be used alone, and two or morekinds thereof may be used in combination.

From the viewpoint of more effectively enhancing the thermal conductionand the adhesiveness and more effectively suppressing variation in theadhesiveness, the thermosetting compound preferably contains an epoxycompound. The epoxy compound is an organic compound having at least oneepoxy group. One kind of the epoxy compound may be used alone, and twoor more kinds thereof may be used in combination.

Examples of the epoxy compound include a bisphenol A type epoxycompound, a bisphenol F type epoxy compound, a bisphenol S type epoxycompound, a phenol novolac type epoxy compound, a biphenyl type epoxycompound, a biphenyl novolac type epoxy compound, a biphenol type epoxycompound, a naphthalene type epoxy compound, a fluorene type epoxycompound, a phenol aralkyl type epoxy compound, a naphthol aralkyl typeepoxy compound, a dicyclopentadiene type epoxy compound, an anthracenetype epoxy compound, an epoxy compound having an adamantane skeleton, anepoxy compound having a tricyclodecane skeleton, a naphthylene ethertype epoxy compound, and an epoxy compound having a triazine nucleus inits skeleton.

From the viewpoint of more effectively enhancing the thermal conductionand the adhesiveness and more effectively suppressing variation in theadhesiveness, the epoxy compound is preferably a bisphenol A type epoxycompound.

From the viewpoint of more effectively enhancing the thermal conductionand the adhesiveness and more effectively suppressing variation in theadhesiveness, the content of the thermosetting compound in 100% byvolume of the insulating sheet is preferably 20% by volume or more andmore preferably 25% by volume or more, and preferably 80% by volume orless and more preferably 75% by volume or less.

(Thermosetting Component: Thermosetting Agent)

For the insulating sheet according to the present invention, athermosetting agent is preferably used together with the thermosettingcompound. The thermosetting agent is not particularly limited. As thethermosetting agent, a thermosetting agent capable of curing thethermosetting compound can be used suitably. Also, as used herein, thethermosetting agent includes a curing catalyst. One kind of thethermosetting agents may be used alone, and two or more kinds thereofmay be used in combination.

Examples of the thermosetting agent include cyanate ester compounds(cyanate ester curing agents), phenolic compounds (phenol thermosettingagents), amine compounds (amine thermosetting agents), thiol compounds(thiol thermosetting agents), imidazole compounds, phosphine compounds,acid anhydrides, active ester compounds, and dicyandiamide. Thethermosetting agent preferably has a functional group capable ofreacting with an epoxy group of the epoxy compound described above.

Examples of the cyanate ester compound include novolac type cyanateester resins, bisphenol type cyanate ester resins, and prepolymersobtained by partially trimerizing those. Examples of the novolac typecyanate ester resins include phenol novolac type cyanate ester resins,and alkylphenol type cyanate ester resins. Examples of the bisphenoltype cyanate ester resins include bisphenol A type cyanate ester resins,bisphenol E type cyanate ester resins, and tetramethyl bisphenol F typecyanate ester resins.

Examples of commercially available products of the cyanate estercompound include phenol novolac type cyanate ester resins (“PT-30” and“PT-60” manufactured by Lonza Japan Ltd.), and prepolymers (“BA-230S,”“BA-3000S,” “BTP-1000S,” and “BTP-6020S” manufactured by Lonza JapanLtd.) obtained by trimerizing bisphenol type cyanate ester resins.

Examples of the phenolic compound include novolac type phenols, biphenoltype phenols, naphthalene type phenols, dicyclopentadiene type phenols,aralkyl type phenols, and dicyclopentadiene type phenols.

Examples of commercially available products of the phenolic compoundinclude novolac type phenols (“TD-2091” manufactured by DICCorporation), biphenyl novolac type phenols (“MEHC-7851” manufactured byMeiwa Plastic Industries, Ltd.), aralkyl type phenolic compounds(“MEH-7800” manufactured by Meiwa Plastic Industries, Ltd.), and phenols(“LA1356” and “LA3018-50P” manufactured by DIC Corporation) having anaminotriazine skeleton.

The total content of the thermosetting compound and the thermosettingagent in 100% by volume of the insulating sheet is preferably 20% byvolume or more and more preferably 25% by volume or more, and preferably80% by volume or less and more preferably 75% by volume or less. Whenthe total content of the thermosetting compound and the thermosettingagent is in the range from the above lower limit to the above upperlimit inclusive, the thermal conduction and the adhesiveness can be moreeffectively enhanced, and variation in the adhesiveness can be moreeffectively suppressed. A content ratio of the thermosetting compoundand the thermosetting agent is appropriately selected so that thethermosetting compound cures.

The content of the thermosetting agent is appropriately selected so thatthe thermosetting compound cures well. The content of the thermosettingagent is preferably 1 part by weight or more and more preferably 3 partsby weight or more, and preferably 50 parts by weight or less and morepreferably 30 parts by weight or less based on 100 parts by weight ofthe thermosetting compound. When the content of the thermosetting agentis more than or equal to the above lower limit, it is more easy tosufficiently cure the thermosetting compound. When the content of thethermosetting agent is less than or equal to the above upper limit, anexcess thermosetting agent that does not contribute to curing is lesslikely to be generated. Thus, heat resistance and adhesiveness of acured product are further enhanced.

(Insulating Filler)

The insulating sheet according to the present invention may contain aninsulating filler. The insulating filler is not the boron nitride. Theinsulating filler has insulation properties. The insulating filler maybe an organic filler or an inorganic filler. One kind of the insulatingfiller may be used alone, and two or more kinds thereof may be used incombination.

From the viewpoint of more effectively enhancing the thermal conduction,the insulating filler is preferably an inorganic filler. From theviewpoint of more effectively enhancing the thermal conduction, theinsulating filler preferably has a thermal conductivity of 10 W/m·K ormore.

From the viewpoint of more effectively enhancing the thermal conduction,the thermal conductivity of the insulating filler is preferably 10 W/m·Kor more and more preferably 20 W/m·K or more. An upper limit of thethermal conductivity of the insulating filler is not particularlylimited. Inorganic fillers having a thermal conductivity of about 300W/m·K are widely known, and inorganic fillers having a thermalconductivity of about 200 W/m·K are readily available.

The material of the insulating filler is not particularly limited.Examples of the materials of the insulating filler include nitrogencompounds (such as boron nitride, aluminum nitride, silicon nitride,carbon nitride, and titanium nitride), carbon compounds (such as siliconcarbide, fluorine carbide, boron carbide, titanium carbide, tungstencarbide, and diamond), and metal oxides (such as silica, alumina, zincoxide, magnesium oxide, and beryllium oxide). The material of theinsulating filler is preferably the nitrogen compound, the carboncompound or the metal oxide, and more preferably alumina, boron nitride,aluminum nitride, silicon nitride, silicon carbide, zinc oxide, ormagnesium oxide. The use of these preferred insulating fillers furtherincreases the thermal conduction of a cured product.

The insulating filler is preferably spherical particles ornon-aggregated particles and aggregated particles having an aspect ratioof more than 2. The use of these insulating fillers further increasesthe thermal conduction of a cured product. The aspect ratio of thespherical particles is 2 or less.

The new Mohs hardness of the material of the insulating filler ispreferably 12 or less and more preferably 9 or less. When the new Mohshardness of the material of the insulating filler is 9 or less, theprocessability of a cured product is further enhanced.

From the viewpoint of further enhancing the processability of a curedproduct, the material of the insulating filler is preferably boronnitride, synthetic magnesite, crystalline silica, zinc oxide, ormagnesium oxide. The new Mohs hardness of the material of each of theseinsulating fillers is 9 or less.

From the viewpoint of further enhancing the thermal conduction, theparticle diameter of the insulating filler is preferably 0.1 μm or moreand 50 μm or less. When the particle diameter of the insulating filleris more than or equal to the above lower limit, the insulating fillercan be easily filled highly densely. When the particle diameter of theinsulating filler is less than or equal to the above upper limit, thethermal conduction of a cured product is further enhanced.

The particle diameter of the insulating filler is preferably an averageparticle diameter obtained by averaging particle diameters in terms ofvolume average measured with a laser diffraction particle sizedistribution measuring apparatus. The average particle diameter of theinsulating filler can also be determined by calculating an average ofthe particle diameters of 50 insulating filler particles selectedrandomly.

From the viewpoint of more effectively enhancing the thermal conduction,the content of the insulating filler in 100% by volume of the insulatingsheet is preferably 1% by volume or more and more preferably 3% byvolume or more, and preferably 30% by volume or less and more preferably25% by volume or less.

(Other Ingredients)

Other than the above-described ingredients, the insulating sheet mayinclude other ingredients, which are generally used for a resin sheetand a curable sheet, such as a dispersant, a chelating agent, and anoxidation inhibitor.

(Laminate)

The laminate according to the present invention includes a thermalconductor, an insulating layer, and a conductive layer. The insulatinglayer is laminated on one surface of the thermal conductor. Theconductive layer is laminated on a surface of the insulating layeropposite to the thermal conductor. The insulating layer may be laminatedalso on the other surface of the thermal conductor. In the laminateaccording to the present invention, the material of the insulating layeris the insulating sheet described above.

Thermal Conductor:

The thermal conductivity of the thermal conductor is preferably 10 W/m·Kor more. As the thermal conductor, an appropriate thermal conductor canbe used. The thermal conductor is preferably a metal material. Examplesof the metal material include metal foil and a metal plate. The thermalconductor is preferably the metal foil or the metal plate and morepreferably the metal plate.

Examples of the material of the metal material include aluminum, copper,gold, silver, a graphite sheet, From the viewpoint of more effectivelyenhancing the thermal conduction, the material of the metal material ispreferably aluminum, copper or gold, and more preferably aluminum orcopper.

Conductive Layer:

The metal for forming the conductive layer is not particularly limited.Examples of the metal include gold, silver, palladium, copper, platinum,zinc, iron, tin, lead, aluminum, cobalt, indium, nickel, chromium,titanium, antimony, bismuth, thallium, germanium, cadmium, silicon,tungsten, molybdenum, and alloys of these. Further examples of the metalinclude tin-doped indium oxide (ITO) and solder. From the viewpoint ofmore effectively enhancing the thermal conduction, the metal ispreferably aluminum, copper or gold, and more preferably aluminum orcopper.

A method of forming the conductive layer is not particularly limited.Examples of the method of forming the conductive layer include a methodby electroless plating, a method by electroplating, and a method ofthermocompression-bonding the insulating layer and metal foil. Themethod of thermocompression-bonding the insulating layer and metal foilis preferable because the conductive layer can be formed in a simplemanner.

FIG. 1 is a cross-sectional view schematically showing an insulatingsheet according to an embodiment of the present invention. Forconvenience of illustration, the size and thickness shown in FIG. 1 aredifferent from the actual size and thickness.

An insulating sheet 1 shown in FIG. 1 contains a thermosetting component11 and boron nitride 12. The boron nitride 12 is preferably theabove-described boron nitride.

The insulating sheet 1 according to the present embodiment has onesurface (first surface) 1 a and the other surface (second surface) 1 b.In the insulating sheet 1 according to the present embodiment, the firstaverage aspect ratio of the boron nitride 12 in a region having athickness of 10% of the thickness of the sheet, from the first surface 1a toward the second surface 1 b is smaller than the second averageaspect ratio of the boron nitride 12 in a region having a thickness of90% of the thickness of the sheet, from the second surface 1 b towardthe first surface 1 a. The first average aspect ratio of the boronnitride 12 in the region having a thickness of 10% of the thickness ofthe sheet, from the first surface 1 a toward the second surface 1 b ispreferably smaller than the third average aspect ratio of the boronnitride 12 in a region having a thickness of 80% of the thickness of thesheet, that is from a position 1/10 of the thickness of the sheet to aposition 9/10 of the thickness of the sheet from the first surface 1 atoward the second surface 1 b.

In the insulating sheet 1 according to the present embodiment, thethermosetting component 11 may contain a thermosetting agent. Thethermosetting component is preferably not completely cured. Thethermosetting component may be B-staged by heating or the like. Thethermosetting component may be a B-staged product.

FIG. 2 is a cross-sectional view schematically showing a laminateobtained using the insulating sheet according to one embodiment of thepresent invention. For convenience of illustration, the size andthickness shown in FIG. 2 are different from the actual size andthickness.

A laminate 21 shown in FIG. 2 includes an insulating layer 22, aconductive layer 23, and a thermal conductor 24. The insulating layer22, the conductive layer 23, and the thermal conductor 24 are theabove-described insulating layer, conductive layer, and thermalconductor. In FIG. 2, the insulating sheet 1 shown in FIG. 1 is used asthe insulating layer 22.

The insulating layer 22 has one surface (first surface) 22 a and theother surface (second surface) 22 b. The conductive layer 23 has onesurface (first surface) 23 a and the other surface (second surface) 23b. The insulating layer 24 has one surface (first surface) 24 a and theother surface (second surface) 24 b.

The conductive layer 23 is laminated on the side of one surface (firstsurface) 22 a of the insulating layer 22. The thermal conductor 24 islaminated on the side of the other surface (second surface) 22 b of theinsulating layer 22. The insulating layer 22 is laminated on the side ofthe other surface (second surface) 23 b of the conductive layer 23. Theinsulating layer 22 is laminated on the side of one surface (firstsurface) 24 a of the thermal conductor 24. The insulating layer 22 isdisposed between the conductive layer 23 and the thermal conductor 24.

The method of producing the laminate is not particularly limited.Examples of the method of producing the laminate include a method inwhich the thermal conductor, the insulating layer, and the conductivelayer are stacked and thermocompression-bonded by vacuum pressing or thelike.

In the laminate 21 according to the present embodiment, the insulatinglayer 22 includes a cured product portion 25 and the boron nitride 12.The insulating layer 22 is formed by the insulating sheet 1 shown inFIG. 1. It is preferable that the insulating layer be formed bythermocompression-bonding the insulating sheet described above by vacuumpressing or the like.

In the laminate 21 according to the present embodiment, the firstaverage aspect ratio of the boron nitride 12 in a region having athickness of 10% of the thickness of the sheet, from the first surface22 a toward the second surface 22 b of the insulating layer 22 issmaller than the second average aspect ratio of the boron nitride 12 ina region having a thickness of 90% of the thickness of the sheet, fromthe second surface 22 b toward the first surface 22 a of the insulatinglayer 22.

In the present embodiment, the cured product portion 1025 is portion inwhich the thermosetting component 11 in FIG. 1 is cured. The curedproduct portion 25 is obtained by curing the thermosetting component 11.The cured product portion 25 may be a portion in which a thermosettingcomponent containing a thermosetting compound and a thermosetting agentis cured.

The insulating sheet can be used in various applications where highthermal conduction, high mechanical strength, and the like are required.For example, the laminate is disposed between a heat generationcomponent and a heat dissipation component to be used in electronicequipment. For example, the laminate is used as a radiator installedbetween a CPU and a fin or a radiator of a power card used in invertersof electric vehicles and the like. Further, the laminate may be used asan insulating circuit board by forming a circuit by etching or the likeof the conductive layer of the laminate.

Hereinafter, the present invention will be clarified by way of specificexamples and comparative examples of the present invention. The presentinvention is not limited to the following examples.

Thermosetting Component (Thermosetting Compound):

(1) “Epicoat 828US” manufactured by Mitsubishi Chemical Corporation,epoxy compound

Thermosetting Component (Thermosetting Agent):

(1) “Dicyandiamide” manufactured by Tokyo Chemical Industry Co., Ltd.

(2) “2MZA-PW” manufactured by Shikoku Chemicals Corporation,isocyanurate-modified solid dispersed imidazole

Boron Nitride:

(1) “HP-40” manufactured by Mizushima Ferroalloy Co., Ltd., averageaspect ratio: 6, average major diameter: 7 μm

(2) “PTX60” manufactured by Momentive Performance Materials Inc.,average aspect ratio: 13, average major diameter: 7 μm

(3) “UHP-G1H” manufactured by SHOWA DENKO K.K., average aspect ratio: 6,average major diameter: 3 μm

(Average Aspect Ratio of Boron Nitride)

The average aspect ratio of boron nitride was measured as follows.

Method of Measuring Average Aspect Ratio of Boron Nitride:

The average aspect ratio of the boron nitride was obtained by mixing theboron nitride or the primary particles constituting the boron nitrideagglomerated particles with thermosetting resin or the like, measuringthe major diameter/minor diameter of each of 50 boron nitride particles(primary particles) randomly selected from an electron microscope imageof a cross section of a sheet or a laminate produced by pressing, andcalculating an average value. When the primary particle had a bentshape, the primary particle was divided into two parts at a bendingpoint, and the major diameter was measured for the two divided parts.The major diameter/minor diameter calculated from the part with a largermajor diameter was taken as the major diameter/minor diameter of theprimary particle.

(Average Major Diameter of Boron Nitride)

The average major diameter of boron nitride was measured as follows.

Method of Measuring Average Major Diameter of Boron Nitride:

The average major diameter of the boron nitride was obtained by mixingthe boron nitride or the primary particles constituting the boronnitride agglomerated particles with thermosetting resin or the like,measuring the major diameter of each of 50 boron nitride particles(primary particles) randomly selected from an electron microscope imageof a cross section of a sheet or a laminate produced by pressing, andcalculating an average value. When the primary particle had a bentshape, the primary particle was divided into two parts at a bendingpoint, and the major diameter was measured for the two divided parts.The major diameter of the part with a larger major diameter was taken asthe major diameter of the primary particle.

Insulating Filler (Alumina):

(1) “A20S” manufactured by SHOWA DENKO K.K., average aspect ratio: 1,average major diameter: 20 μm, particle diameter: 20 μm

Example 1

(1) Preparation of Curable Materials A and B

A thermosetting compound and a thermosetting agent were blended suchthat based on 100 parts by weight of “Epicoat 828US” manufactured byMitsubishi Chemical Corporation, “Dicyandiamide” manufactured by TokyoChemical Industry Co., Ltd. had an amount of 10 parts by weight, and“2MZA-PW” manufactured by Shikoku Chemicals Corporation had an amount of1 part by weight. Next, boron nitride indicated in Table 1 below wasblended in the blending amount (% by volume) indicated in Table 1 below,and stirred with a planetary stirrer at 500 rpm for 25 minutes to obtainthe curable materials A and B.

(2) Production of Laminate

The obtained curable material A was coated on a release PET sheet (50 μmthick) to have a thickness of 100 μm and dried in an oven at 50° C. for20 minutes to form a first curable material layer. Next, the obtainedcurable material B was coated on the first curable material layer tohave a thickness of 250 μm and dried in an oven at 50° C. for 20 minutesto form a second curable material layer, and thus to obtain aninsulating sheet. The obtained insulating sheet has a first surfacewhich is the surface of the first curable material layer formed, and asecond surface which is the surface of the second curable material layerformed.

Thereafter, the release PET sheet was peeled off, copper foil wasstacked on the first surface of the insulating sheet, and an aluminumplate was stacked on the second surface of the insulating sheet. Thestacked layers were vacuum-pressed at a temperature of 200° C. and apressure of 10 MPa to produce a laminate. That is, the obtained laminateincludes a thermal conductor, an insulating layer laminated on onesurface of the thermal conductor, and a conductive layer laminated on asurface of the insulating layer opposite to the thermal conductor, and amaterial of the insulating layer is the obtained insulating sheet. Thethickness of the insulating layer of the obtained laminate was 245 μm.

Example 2

In the same manner as in Example 1, curable materials A and B wereobtained. The obtained curable material A was coated on a release PETsheet (50 μm thick) to have a thickness of 50 μm and dried in an oven at50° C. for 20 minutes to form a first curable material layer. Next, theobtained curable material B was coated on the first curable materiallayer to have a thickness of 200 μm and dried in an oven at 50° C. for20 minutes to form a second curable material layer. Furthermore, theobtained curable material A was coated on the second curable materiallayer to have a thickness of 50 μm and dried in an oven at 50° C. for 20minutes to form a third curable material layer, and thus to obtain aninsulating sheet. The obtained insulating sheet has a first surfacewhich is the surface of the first curable material layer formed, and asecond surface which is the surface of the third curable material layerformed.

Thereafter, the release PET sheet was peeled off, copper foil wasstacked on the first surface of the insulating sheet, and an aluminumplate was stacked on the second surface of the insulating sheet. Thestacked layers were vacuum-pressed at a temperature of 200° C. and apressure of 10 MPa to produce a laminate. That is, the obtained laminateincludes a thermal conductor, an insulating layer laminated on onesurface of the thermal conductor, and a conductive layer laminated on asurface of the insulating layer opposite to the thermal conductor, and amaterial of the insulating layer is the obtained insulating sheet. Thethickness of the insulating layer of the obtained laminate was 245 μm.

Example 3

A laminate was obtained in the same manner as Example 1, except that: inthe process of producing the curable material A, “A20S” was blended, andthe blending amount of “A20S” was set to 10% by volume; and in theprocess of producing the curable material B, “A20S” was blended, and theblending amount of “A20S” was set to 10% by volume. The thickness of theinsulating layer of the obtained laminate was 245 μm.

Example 4

A laminate was obtained in the same manner as Example 2, except that: inthe process of producing the curable material A, “A20S” was blended, andthe blending amount of “A20S” was set to 10% by volume; and in theprocess of producing the curable material B, “A20S” was blended, and theblending amount of “A20S” was set to 10% by volume. The thickness of theinsulating layer of the obtained laminate was 245 μm.

Example 5

A laminate was obtained in the same manner as Example 1, except that inthe process of producing the curable material A, “A20S” was blended, andthe blending amount of “A20S” was set to 10% by volume. The thickness ofthe insulating layer of the obtained laminate was 245 μm.

Example 6

A laminate was obtained in the same manner as Example 2, except that inthe process of producing the curable material A, “A20S” was blended, andthe blending amount of “A20S” was set to 10% by volume. The thickness ofthe insulating layer of the obtained laminate was 245 μm.

Example 7

A laminate was obtained in the same manner as Example 1, except that inthe process of producing the curable material B, “A20S” was blended, andthe blending amount of “A20S” was set to 10% by volume. The thickness ofthe insulating layer of the obtained laminate was 245 μm.

Example 8

A laminate was obtained in the same manner as Example 2, except that inthe process of producing the curable material B, “A20S” was blended, andthe blending amount of “A20S” was set to 10% by volume. The thickness ofthe insulating layer of the obtained laminate was 245 μm.

Example 9

A laminate was obtained in the same manner as Example 1, except thatboron nitride was changed from “HP-40” to “UHP-G1H” in the process ofproducing the curable material A. The thickness of the insulating layerof the obtained laminate was 245 μm.

Example 10

A laminate was obtained in the same manner as Example 2, except thatboron nitride was changed from “HP-40” to “UHP-G1H” in the process ofproducing the curable material A. The thickness of the insulating layerof the obtained laminate was 245 μm.

Example 11

A laminate was obtained in the same manner as Example 1, except that: inthe process of producing the curable material A, boron nitride waschanged from “HP-40” to “UHP-G1H”, “A20S” was blended, and the blendingamount of “A20S” was set to 10% by volume; and in the process ofproducing the curable material B, “A20S” was blended, and the blendingamount of “A20S” was set to 10% by volume. The thickness of theinsulating layer of the obtained laminate was 245 μm.

Example 12

A laminate was obtained in the same manner as Example 2, except that: inthe process of producing the curable material A, boron nitride waschanged from “HP-40” to “UHP-G1H”, “A20S” was blended, and the blendingamount of “A20S” was set to 10% by volume; and in the process ofproducing the curable material B, “A20S” was blended, and the blendingamount of “A20S” was set to 10% by volume. The thickness of theinsulating layer of the obtained laminate was 245 μm.

Example 13

A laminate was obtained in the same manner as Example 1, except that inthe process of producing the curable material A, boron nitride waschanged from “HP-40” to “UHP-G1H”, “A20S” was blended, and the blendingamount of “A20S” was set to 10% by volume. The thickness of theinsulating layer of the obtained laminate was 245 μm.

Example 14

A laminate was obtained in the same manner as Example 2, except that inthe process of producing the curable material A, boron nitride waschanged from “HP-40” to “UHP-G1H”, “A20S” was blended, and the blendingamount of “A20S” was set to 10% by volume. The thickness of theinsulating layer of the obtained laminate was 245 μm.

Example 15

A laminate was obtained in the same manner as Example 1, except that:boron nitride was changed from “HP-40” to “UHP-G1H” in the process ofproducing the curable material A; and in the process of producing thecurable material B, “A20S” was blended, and the blending amount of“A20S” was set to 10% by volume. The thickness of the insulating layerof the obtained laminate was 245 μm.

Example 16

A laminate was obtained in the same manner as Example 2, except that:boron nitride was changed from “HP-40” to “UHP-G1H” in the process ofproducing the curable material A; and in the process of producing thecurable material B, “A20S” was blended, and the blending amount of“A20S” was set to 10% by volume. The thickness of the insulating layerof the obtained laminate was 245 μm.

Comparative Example 1

A laminate was produced in the same manner as Example 1, except thatboron nitride was changed from “HP-40” to “PTX60” in the process ofproducing the curable material A, the curable material B was notproduced, and the curable material A was used instead of the curablematerial B. The thickness of the insulating layer of the obtainedlaminate was 245 μm.

Comparative Example 2

A laminate was produced in the same manner as Example 1, except that thecurable material B was not produced, and the curable material A was usedinstead of the curable material B. The thickness of the insulating layerof the obtained laminate was 245 μm.

(Evaluation)

(1) Average Aspect Ratio of Boron Nitride

The cross section of the obtained laminate was processed for smoothingby a cross section polisher (“IB-19500CP” manufactured by JEOL Ltd.),and the cross section of the processed laminate was observed by a fieldemission scanning electron microscope (“S-4800” manufactured by HitachiHigh-Technologies Corporation). Boron nitride was identified by anenergy dispersive X-ray spectrometer (“S-4800EN-EDS” manufactured byHitachi High-Technologies Corporation). The first average aspect ratioof boron nitride in the region having a thickness of 10% of thethickness of the sheet, from the first surface of the insulating sheettoward the second surface of the insulating sheet and the second averageaspect ratio of boron nitride in the region having a thickness of 90% ofthe thickness of the sheet, from the second surface of the insulatingsheet toward the first surface of the insulating sheet were calculatedfrom the obtained electron microscope image. Further, the third averageaspect ratio of the boron nitride in the region having a thickness of80% of the thickness of the sheet, that is from the position 1/10 of thethickness of the sheet to the position 9/10 of the thickness of thesheet from the first surface of the insulating sheet toward the secondsurface of the insulating sheet was calculated. Furthermore, the fourthaverage aspect ratio of boron nitride in the region having a thicknessof 10% of the thickness of the sheet, from the second surface of theinsulating sheet toward the first surface of the insulating sheet wascalculated.

(2) Thermal Conductivity

The obtained laminate was cut into 1 cm squares, and then carbon blackwas sprayed on both sides to prepare a measurement sample. The thermalconductivity was calculated by a laser flash method using the obtainedmeasurement sample. A relative value obtained when the value ofComparative Example 1 was expressed as 1.0 was calculated, and thethermal conductivity was judged on the basis of the following criteria.

[Criteria for Judgment in Thermal Conductivity]

◯◯: thermal conductivity was 1.5 or more.

◯: thermal conductivity was more than 1.0 and less than 1.5.

Δ: Comparative Example 1 (1.0), or thermal conductivity was equivalentto Comparative Example 1 (1.0).

x: thermal conductivity was less than 1.0.

(3) 90 Degree Peel Strength

The obtained laminate was cut out to the size of 50 mm×120 mm to obtaina Lest sample. Copper foil was peeled off so that copper foil with awidth of 10 mm was left in the center of the obtained test sample, andthe peel strength of the copper foil was measured according to JIS C6481 with respect to the copper foil with a width of 10 mm in thecenter. As a peel strength tester for measuring the peel strength, a“Tensilon universal testing machine” manufactured by Orientec K.K. wasused. For 20 test samples, the peel strength of the copper foil wasmeasured to obtain 20 measured values of 90 degree peel strength. Anaverage value of the 20 measured values of the 90 degree peel strengthwas taken as the 90 degree peel strength. A relative value obtained whenthe value of Comparative Example 1 was expressed as 1.0 was calculated,and the 90 degree peel strength was judged on the basis of the followingcriteria.

[Criteria for Judgment in 90 Degree Peel Strength]

◯◯: 90 degree peel strength was 1.5 or more.

◯: 90 degree peel strength was more than 1.0 and less than 1.5.

Δ: Comparative Example 1 (1.0), or 90 degree peel strength wasequivalent to Comparative Example 1 (1.0).

x: 90 degree peel strength was less than 1.0.

(4) Variation in Adhesiveness

In the evaluation of (3), variation in adhesiveness was judged on thebasis of the following criteria from 20 measured values of the 90 degreepeel strength.

[Criteria for Judgment in Variation in Adhesiveness]

◯◯: a difference between a maximum value and a minimum value of the 90degree peel strength was less than 0.3 N/10 mm.

◯: the difference between the maximum value and the minimum value of the90 degree peel strength was 0.3 N/10 mm or more and less than 0.7 N/10mm.

x: the difference between the maximum value and the minimum value of the90 degree peel strength was 0.7 N/10 mm or more.

Tables 1 to 3 below show the configurations and results of theinsulating sheet.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Curable Amount (vol %) of HP-40 55 55 55 55 55 55 material A boronnitride PTX60 UHP-G1H Amount (vol %) of A20S 10 10 10 10 alumina CurableAmount (vol %) of HP-40 material B boron nitride PTX60 55 55 55 55 55 55Amount (vol %) of A20S 10 10 alumina Content (vol %) of boron nitride in100 vol % 55 55 55 55 55 55 of insulating sheet Evaluation Averageaspect First average 6 6 6 6 6 6 ratio of boron aspect ratio nitrideSecond average 12.5 12.5 12.5 12.5 12.5 12.5 aspect ratio Third average12.3 12.1 12.3 12.1 12.3 12.1 aspect ratio Fourth average 13 6 13 6 13 6aspect ratio Thermal conductivity ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ 90 degree peelstrength ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ Variation in adhesiveness ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘

TABLE 2 Example Example Example Example 7 Example 8 Example 9 10 11 12Curable Amount (vol %) of HP-40 55 55 material A boron nitride PTX60UHP-G1H 55 55 55 55 Amount (vol %) of A20S 10 10 alumina Curable Amount(vol %) of HP-40 material B boron nitride PTX60 55 55 55 55 55 55 Amount(vol %) of A20S 10 10 10 10 alumina Content (vol %) of boron nitride in100 vol % 55 55 55 55 55 55 of insulating sheet Evaluation Averageaspect First average 6 6 6 6 6 6 ratio of boron aspect ratio nitrideSecond average 12.5 12.5 12.5 12.5 12.5 12.5 aspect ratio Third average12.3 12.1 12.3 12.1 12.3 12.1 aspect ratio Fourth average 13 6 13 6 13 6aspect ratio Thermal conductivity ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ 90 degree peelstrength ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ Variation in adhesiveness ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘

TABLE 3 Example Example Example Example Comparative Comparative 13 14 1516 Example 1 Example 2 Curable Amount (vol %) HP-40 55 material A ofboron PTX60 55 nitride UHP-G1H 55 55 55 55 Amount (vol %) A20S 10 10 ofalumina Curable Amount (vol %) HP-40 material B of boron PTX60 55 55 5555 nitride Amount (vol %) A20S 10 10 of alumina Content (vol %) of boronnitride in 55 55 55 55 55 55 100 vol % of insulating sheet EvaluationAverage First 6 6 6 6 13 6 aspect ratio average of boron aspect rationitride Second 12.5 12.5 12.5 12.5 13 6 average aspect ratio Third 12.312.1 12.3 12.1 13 6 average aspect ratio Fourth 13 6 13 6 13 6 averageaspect ratio Thermal conductivity ∘∘ ∘∘ ∘∘ ∘∘ Δ x 90 degree peelstrength ∘∘ ∘∘ ∘∘ ∘∘ Δ ∘ Variation in adhesiveness ∘∘ ∘∘ ∘∘ ∘∘ x ∘

EXPLANATION OF SYMBOLS

-   -   1: Insulating sheet    -   1 a: One surface (first surface)    -   1 b: The other surface (second surface)    -   11: Thermosetting component    -   12: Boron nitride    -   21: Laminate    -   22: Insulating layer    -   22 a: One surface (first surface)    -   22 b: The other surface (second surface)    -   23: Conductive layer    -   23 a: One surface (first surface)    -   23 b: The other surface (second surface)    -   24: Thermal conductor    -   24 a: One surface (first surface)    -   24 b: The other surface (second surface)    -   25: Cured product portion (portion in which thermosetting        component is cured)

The invention claimed is:
 1. An insulating sheet comprising: athermosetting component; and boron nitride, the insulating sheet havinga first surface on one side in a thickness direction and a secondsurface on the other side in the thickness direction, wherein a firstaverage aspect ratio of the boron nitride in a region having a thicknessof 10% of a thickness of the sheet, from the first surface toward thesecond surface, is smaller than a third average aspect ratio of theboron nitride in a region having a thickness of 80% of the thickness ofthe sheet, that is from a position 1/10 of the thickness of the sheet toa position 9/10 of the thickness of the sheet from the first surfacetoward the second surface, and a fourth average aspect ratio of theboron nitride in a region having a thickness of 10% of the thickness ofthe sheet, from the second surface toward the first surface, is smallerthan the third average aspect ratio.
 2. The insulating sheet accordingto claim 1, wherein the first average aspect ratio is 2 or more and 20or less.
 3. The insulating sheet according to claim 1, wherein anaverage major diameter of the boron nitride of the entire insulatingsheet is 1 μm or more and 40 μm or less.
 4. The insulating sheetaccording to claim 1, wherein a content of the boron nitride in theentire insulating sheet is 20% by volume or more and 80% by volume orless in 100% by volume of the entire insulating sheet.
 5. A laminatecomprising: a thermal conductor; an insulating layer laminated on onesurface of the thermal conductor; and a conductive layer laminated on asurface of the insulating layer opposite to the thermal conductor,wherein a material of the insulating layer is the insulating sheetaccording to claim 1.